Compact arc pressure lamp with internal gas circulation through cathodic plasma jet action



w. E. THoURx-:T ETAL 3,474,278

Oct. 2l, 1969 COMPACT ARC PRESSURE LAMP WITH INTERNAL GAS CIRCULATIONTHROUGH CATHODIC PLASMA JET ACTION Filed Sept.

United States Patent OA U.S. Cl. 313-231 18 Claims ABSTRACT F THEDISCLOSURE A high wattage, compact arc high pressure discharge lamphaving a rare gas and/or metal vapor filling using internal gascirculation by cathodic plasma jet action to produce cooling.

High pressure compact arc type (i.e. arc electrodes placed relativelyclose together rather than at the ends of an elongated tube) dischargelamps with either rare gas or metal vapor filling, have recently foundincreased usage in optical equipment. Typical applications include theuse of these lamps as light or radiation sources for searchlights,projectors, and devices that simulate the suns radiation in outer spacefor the purpose of testing space vehicles or their components. Lamps ofthis type arervery suitable for such equipment because they provide astable source of radiation with a very high brightness and good luminousefficacy. They also operate cleanly without adjustment or maintenancefor hundreds or even thousands, of hours.

Because of their many advantages, high pressure compact arc-type lampsare increasingly replacing carbon arc type lamps which were previouslyused in many of the aforementioned applications. Since carbon arc lampsare available or can be -built for input Wattage ratings up to 20, 50 oreven 100 kilowatts, it is desirable to provide compact arc dischargelamps having similar high wattage ratings. This demand has been partlymet through development of rare gas (xenon) high pressure compact arclamps having input ratings in the -30 kilowatt range. Lamps of thisgeneral type are described in an article entitled High Brightness XenonLamps with Liquid-Cooled Electrodes by W. E. Thouret, H. S. Strauss, S.F. Cortorillo, H. Kee, in Illuminating Engineering, vol. 60 page 399,1965.

For lamps with a rare gas filling, such as xenon, an input wattagerating exists above which economic and reliable lamp designs cannot beobtained without using forced cooling of the electrodes. This limitpresently seems to lie somewhere between 10 and 2,0 kilowatts, dependingupon the particular conditions of the application for which the lamp isto be used. The use of forced cooling of the electrodes increases thewattage rating of compact arc type lamps to a considerable degree. Forexample, high brightness xenon lamps in the 20 to 30 kilowatt inputrange have been developed using liquid cooled electrodes and seals. Suchlamps operate economically and reliably, because due to the liquidcooling, quartz-to-metal seals of relatively simple construction can beused. The electrodes of these duid-cooled lamps 3,474,278 Patented Oct.2l, 1969 are capable of carrying currents up to 1000 amperes and theyhave practically unlimited life because they operate at only slightlyelevated temperatures. One such liquid cooled lamp is described in thecopending application of W. E. Thouret, S.N. 586,065 filed Oct. l2, 1966entitled Compact Arc High Pressure Discharge Lamp with Liquid CooledElectrodes and Halogen Cycle, and assigned to the same assignee.

The lives of lamps with uncooled seals and electrodes are usuallyterminated by seal failure, because the seals have to be operated at atemperature of several hundred degrees centigrade. In general it can besaid that liquid cooled electrodes are highly advantageous for largewattage compact arc lamps because they allow both the use of smallerbulbs for the same wattage as an uncooled lamp and the use of simplifiedhigh current seals. However, even when liquid cooled electrodes areused, the bulb dimensions required for reliable, long lamp life designsbecome quite large when the total lamp input eX- ceeds 20 kilowatts. Ithas been found, for example, that in a compact arc lamp with liquidcooled electrodes the quartz bulb cannot be loaded to more thanapproximately 45 watts/cm.2 or 280 watts/sq. in. of the lamp inputwattage. If loaded higher, the operating temperature of the quartz bulbincreases into the range in which structural changes occur fast enoughin fused quartz to reduce the mechanical strength of the envelope duringthe expected useful life of the lamp. The term structural changes isused to mean all changes that lead to a decrease in tensile strength,for example, local recrystallization.

In general it has been found that the bulb operating temperature of acompact arc discharge lamp and, consequently, the applicable specificbulb surface loading in watts/cm.2 is determined by that part of thelamp input power (wattage which is the product of lamp current andapplied voltage) that is not converted into radiation for which the lampbulb is transparent. More or less rough estimates and calculations leadto the following distribution of the input energy for high wattagecompact arc lamps with quartz bulb and liquid cooled electrodes:

Total input wattage Due to the inherent properties of compact arc lampdesign, the ultimate disposition of the input energy in items (a) to (d)cannot be changed to any significant degree through modification of thelamp internal construction. Therefore, item (e) appears to present themost promising area in which improvement resulting in higher surfaceloading can be achieved.

The energy balance table given above indicates that only items (b), (d),and (e) contribute energy that remains within the bulb and causes itstemperature to rise. These three items amount in total to about 25% ofthe lamp input. As seen, 60% of the bulb heating energy of items (b),(d) and (e) is supplied by item (e), the energy transferred from the arcto the bulb by gas convection If a part of this convection energy can beremoved frorn the lamp before it reaches the bulb, the specie surfaceloading of the bulb in watts/ cm.2 can be increased. For example, if 80%of the gas convection energy can be removed the surface loading of agiven bulb can be raised substantially, and perhaps be nearly doubled.This means that, for example, the bulb size that now is suflicient forreliable operation of a 30 kw. input lamp would be satisfactory for alamp with 50 to 60 kw. input.

For the purpose of removing the energy carried through gas convectionfrom the extremely hot plasma (in the order of 6000 to 10000 K.) to thelamp bulb, it previously has been suggested to circulate the gas throughan external vacuum-tight and pressurized heat exchange system. Whilesuch gas circulating systems operate to a measure to produce the desiredresult they have several disadvantages. For example, they are quiteintricate and they considerably add to the complications and serviceproblems connected with the operation of high powered arc lamps withliquid cooled electrodes.

The present invention provides a compact arc highpressure lamp in whichprovisions are made to remove considerably more heat from the inner lampvolume than is possible through the use of water-cooled electrodes ofconventional design. Thus the quartz bulb can be loaded higher andeither smaller, less costly bulbs can be used for the same Wattage ormuch higher Wattages can be applied without increasing the bulb size.

In accordance with the invention, the hot plasma gas is circulatedthrough a channel, or channels provided in one or both of theelectrodes, by means of the cathodic plasma jet inherent to directcurrent high wattage compact arcs.

The plasma gas circulation is achieved within the lamp, without the useof an external pump and/or heat exchange system, through utilization ofthe cathodic plasma jet. As is well known to those skilled in the artthe cathodic plasma jet provides powerful acceleration of the plasma gasin a direction from the cathode toward the anode parallel to the arcaxis independent of the lamps burning position. The gas in the plasmajet reaches typical velocities of 500 to 1000 feet/second. By using theplasma jet action, in accordance with the present invention, a largeportion of the energy (as much as 80%) usually transferred from the arcto the bulb by gas convection is taken up by a cooling liquid which ispreferably circulated through the anode. This as explained previously,permits an increase in the surface loading of the bulb.

It is therefore an object of the present invention to provide a compactarc type discharge lamp in which a portion of the plasma is cooled.

A further object is to provide a compact arc type discharge lamp havingan anode construction which permits the circulation of heated gas tocome into contact with a cooling medium through cathodic jet action andconstricts the size of the arc.

Another object is to provide a compact arc type discharge lamp in whichcathodic jet action of the plasma is utilized to cool the gas within thelamp.

An additional object is to provide a compact arc type discharge lamphaving novel constructions for the anode and cathode electrodes whichconstrict the arc size and enhance cooling of the gas within the tube.

Other objects and advantages of the present invention will become moreapparent upon consideration of the following specification and annexeddrawings in which:

FIG. 1 is a cross-sectional view of one embodiment of lamp made inaccordance with the present invention.

FIG. 1A is a cross-sectional view of the anode portion of the lamp ofFIG. l, turned FIG. 2 is a cross-sectional View of the cathode portionof another embodiment of lamp made in accordance with the presentinvention.

FIGS. 1 and 1A show in cross-section a high pressure rare gas or metalvapor filled high, wattage compact arc lamp designed according to thisinvention. The lamp has a spherical or nearly spherical bulb 1 with twointegral, generally cylindrical extensions 12 and 13. The bulb 1 and itsextensions are preferably made of a suiatble refractory, transparent ortranslucent material such as quartz.

Mounted within the respective extensions 12 and 13 are the anode andcathode electrodes 16 and 17 which are generally cylindrical in shapeand are hollow for a substantial portion of their lengths. Theelectrodes 16 and 17 are preferably made as thick walled tubes ofmolybdenum, nickel, stainless steel or a similar suitable metal. Currentis supplied to each electrodes 16 and 17 by a suitable cable (not shown)and a fluid supply and return (not shown) are also connected to theouter ends of the electrodes.

The anode electrode 16 has a tip 18 thereon with a rounded outer edge19. The cathode electrode 17 has a tip 20 with a pointed end 21. Bothtips 18 and 20 are made of massive tungsten or another suitablerefractory metal. The anode tip 18 is brazed vacuum-tight to the anodetube 16 at 22 while the cathode tip 20 is riveted to the cathode tube 17at 24. The cathode tip has a shouldered down portion 25 whichaccommodates the rivet 24. The two tips 18 and 20 of the electrodes aredisposed opposite each other so that an arc is struck therebetween whencurrent is applied to the electrodes.

The electrodes 16 and 17 are sealed, vacuum-tight, to the bulb byreentrant, or folded back, portions 28 of the extensions 12 and 13 andcups 29, which are of a suitable material such as Kovar. The reduceddiameter portions of each of the cups 29 is sealed to the electrodewhile the larger diameter portion is sealed to the end of an area 30 ofgraded glass seals which extends from reentrant portions 28 to the endof the cup. The graded seals comprise a plurality of different glasseswith graded expansion coefficients. The purpose of these seals is tobridge the difference between the expansion coefcients of the cupmaterial and the bulb material.

A lateral support 31 is anchored within the bulb Wall near the end ofeach electrode tube 16 and 17. These supports firmly hold and locate theinner ends of the electrodes and prevent any bending stresses within theglass seals. The supports are made of a suitable material, such astungsten. A gettering material such as a piece of tantalum or titaniumsheet 34 is fastened around the shank of the cathode tip as a gettersubstance to collect certain impurities which are produced during lampmanufacture and operation.

As shown in FIGS. l and 1A, the anode tip 18 is hollow or ring-shapedand has an axially located hole 35. The hole 35 communicates with apassage formed by a tube 36 of metal in the end of anode electrode tube16. Tube 36 branches out into two metal tubes 37 and 38 which aregenerally at right angles, to form a generally T-shaped passagewaycommunicating with the hole 3S in the anode tip.

As shown more clearly in FIG. 1A, the interior of the anode tube 16 issupplied with cooling fluid from a conduit 40 which branches out neartubes 37 and 38 into two conduits 41. The end of anode electrode tube 16is sealed by the tip 18 and tube 36. The cooling fluid returns to itssource through the portion of the anode tube surrounding the coolingconduits. The walls of tubes 36, 37, 38 are preferably thin to affordthe maximum amount of heat transfer and their walls are completelysurrounded by the cooling uid from conduit 40. The cooling fluid sourceand return connections are not shown since they are conventional in theart.

When the lamp is operated an arc is struck between the opposing tips ofthe electrodes and a hot plasma lstream is produced. This is shown bythe lines 40 in FIG. 1. The hole 35 in the anode tip 18 leads the hotplasma stream into the thin walled T-shaped metal tube 36, 37, 38 whichis completely surrounded by the cooling iiuid that cools the anode tip18 (see FIG. 1A). The connections between anode tip 18, T-shaped tubes36, 37, 38 and main anode tube 16 are all made vacuum-tight throughvacuum-brazing, electron beam, welding or other suitable processes. TheT-tube 36, 37, 38 is so dimensioned that the fast moving plasma gasstream becomes turbulent and loses a large portion of its heat to thesurrounding ffluid cooled metal walls thereby reducing the temperatureof the stream as it leaves the passages formed by tubes 37 and 38. Asindicated in FIG. 1 by the arrows, the cooled gas leaves the anodethrough the openings of the T-tube, circulates across the bulb into thecathode region and there is drawn into the cathodic jet, which completesthe cycle. The cathodic jet is produced at the cathode tip due tomagnetic constriction of the hot gases. This jet can have a highvelocity, in the order of 500 to 1000 feet per second. The diameter ofthe anode tip hole 35 is preferably made approximately 1A to l/z of thediameter of the arc plasma which would be produced if a conventionalanode Without hole were utilized.

A feature of the design of the anode electrode according to thisinvention and as shown in FIGS. 1 and 1A is the cooling of the hotplasma gas before it reaches the bulb. This cooling removes aconsiderable part of the energy that is otherwise carried from the arcdirectly to the bulb by convection. In addition, the axial hole 35 inthe anode constricts the arc since it provides a more ready path for thearc to travel than the portion of the anode surrounding the hole. Thisproduces a considerable increase of the arc brightness and radiance inthe anode region. This is a desirable improvement as the averagebrightness of compact arc lamps operating on direct current` withconventional anodes is relatively low because the anode region is quitediffuse in comparison to the extremely constricted cathode region.

In addition, through the use of a hollow anode, the arc plasma isconsiderably more concentrated in the anode Vicinity and the averagebrightness of the arc is substantially increased. Also, the arc isperfectly centered with respect to the cathode-anode axis independent ofthe lamps position in relation to gravity.

The constriction of the arc can be increased further through the use ofa cathode 47 according to FIG. 2. This cathode is designed to operatewith the anode of FIGS. 1 and 1A. The tip 48 of this cathode is pointedbut is relatively short and small. It is brazed or welded to the end ofthe cathode tube sealing it with respect to cooling fluid which issupplied from a source (not shown) through an entrance conduit `50. Thefluid returns through the space between the inner wall of electrode tube47 and the outer wall of conduit 50. The tip `48 is cooled by thecooling liquid spraying from the end of the entrance conduit 50. Gettermaterial, formed by several windings of titanium or tantalum wire, isprovided on the tip 48.

The cathode tube 47 is formed with a shank portion 52 which is directlycooled by the fluid from conduit 50'. The shank 52 has a cylinder 54thereon which is spaced from the shank and which has several openings55. In this arrangement the circulating plasma gas can enter openings 55and flow directly over the liquid cooled cathode shank 52 until itenters the cathodic plasma jet at the cathode tip 48 (as indicated byarrows). By using this cathode design a part of the circulating gas flowis accelerated and precooled directly by the cathode before entering thecathodic jet. This causes an additional constriction of the arc over itsentire length and an increase of brightness and radiance, when comparedto arcs oper# ating from a conventional cathode as shown in FIG. 1 underotherwise equal conditions.

The lamps of the subject application can use conventional iills, e.g. 3to 18 atmospheres of xenon, such as described in my Patent No.2,974,249. They also can use a halogen additive as described in theaforementioned application. Other additives can be utilized, if desired,to vary the color of the plasma, i.e. use gases which ionize more orless readily.

While preferred embodiments of the invention have been described above,it will be understood that these are illustrative only, and theinvention is limited solely by the appended claims:

1. A compact arc discharge lamp comprising an outer envelope, a pair ofelectrode bodies to which electrical current is to be applied mountedwithin said envelope with the ends of said electrode bodies beingoppositely disposed to produce an arc therebetween, and at least one ofsaid electrode bodies formed with a passage having an entrance and anexit end within the interior confines of the envelope for circulatinghot gas therethrough within the interior of the envelope.

2. A lamp as set forth in claim 1 wherein the hot gas circulated is theplasma gas formed by the arc discharge.

3. A compact arc discharge lamp comprising an outer envelope, a pair ofelectrode bodies to which electrical current is to be applied mountedwithin said envelope with the ends of said electrode. bodies beingoppositely disposed to produce an arc discharge therebetween, one ofsaid bodies formed with a passage having an entrance end and an exit endcommunicating with the interior of said envelope through which the hotgas produced by the arc discharge circulates within the envelope, andmeans for cooling the gas circulating through said passage.

4. A lamp as set forth in claim 3 wherein said cooling means comprisesmeans for circulating uid within said body in a heat exchangerelationship with the gas circulating through said passage.

5. A lamp as set forth in claim 2 wherein said electrode bodies comprisean anode and cathode, said gas-circulating passage formed in the anodeelectrode, the entrance end of said passage being in the line of the arcdischarge produced between the anode and cathode electrodes.

6. A lamp as set forth in claim 5 wherein the entrance end of saidpassage for the gas lies opposite the cathode electrode and isconstructed to constrict the arc.

7. A lamp as in claim 5 wherein said passage includes at least one exitpassage in the anode which conducts the gas flow away from the anode.

8. A lamp as set forth in claim 7 wherein the passage is generallyT-shaped.

9. A lamp as set forth in claim 7 wherein the exit passage conducts thecirculated gas toward the wall of the envelope.

10. A lamp as set forth in claim S further comprising a passage formedin said cathode having an entrance end and an exit end communicatingwith the interior of the envelope through which the plasma gas also cancirculate.

11. A lamp as set forth in claim 3 further comprising a passage in theother of said electrode bodies, said passage having an entrance end andan exit end which are both in communication with the interior of theenvelope through which the hot gas produced by the arc discharge alsocirculates Within the envelope.

12. A lamp as set forth in claim 11 further comprising means for coolingthe gas circulating through the passage of said other body.

13. A lamp as set forth in claim 10 wherein the cathode passage directsthe gas ow from the cathode toward the anode passage.

14. A lamp as set forth in claim 12 wherein said cooling means for thegas circulating in the passage of the other body comprises means forcirculating uid within said other body in a heat exchange relationshipwith the gas circulating the said passage of said other body.

15. A lamp as set forth in claim 3 wherein said electrode bodiescomprise an anode and cathode, said gascirculating passage formed in theanode electrode, the 5 entrance end of said passage being in the line ofthe arc discharge produced between the anode and cathode electrodes.

16. A lamp as set forth in claim 15 wherein the entrance end of saidpassage for the gas lies opposite the cathode electrode and isconstructed to constrict the arc.

17. A lamp as in claim 15 wherein said passage includes at least oneexit passage in the anode which conducts the gas flow away from theanode.

18. A lamp as set forth in claim 17 wherein the passage is generallyT-shaped.

JAMES W. LAWRENCE, Primary Examiner 0 RAYMOND F. HOSSFELD, AssistantExaminer U.S. C1. X.R.

